Abstract Engine (MiniMina) for integration tests

[joaosreis]

This PR introduces the abstract engine for integration testing and general improvements.

The general idea is that the abstract engine should be agnostic to the network runner. It operates by using network runner-specific config files (e.g. the mock config located in ./integration_tests/) and running bash commands based on the config. The stdout of the network runner commands is captured by the abstract engine and parsed into the response types defined in config_util.ml, which are subsequently used to manage Lucy's internal network state. Network runner responses must conform to return_spec.json. Node interactions still proceed via GQL.

The other major departures from the GCP engine are the abstract engine's generation of a topology file to be consumed by the network runner and the usage of pre-generated network and libp2p keys to boost performance.

• adds abstract engine module in src/lib/integration_test_abstract_engine
• adds topology file generation
• adds usage of pre-generated network and libp2p keys
• adds config and return spec

[joaosreis]

!ci-build-me